The present invention relates to an improved foamable composition, and a method using the improved composition to form a foamed phenolic material having improved surface skin and cell structure characteristics.
It is well known that foamed phenolic materials can be made by mixing a liquid phenol-formaldehyde resin or resole, an acidic curing agent, a foaming agent, a surface active agent and, if desired, one or more filler or extender materials, and foaming the resulting mixture to the desired configuration, whereafter it is cured. Such foamed phenolic materials are useful for various purposes including thermal insulation, acoustical packaging, filling material for voids or hollow spaces, packaging material for making moldings and as a replacement for such low density materials as balsa wood and low density urethanes. In these uses, however, foamed phenolic materials having superior surface skin characteristics, and a uniform and controllable density, are greatly desired.
In such foam systems, the curing agent, or catalyst, initiates the exothermic curing reaction of the resin. The heat given off by this exothermic reaction vaporizes the foaming agent, causing bubbles of gas to be formed in situ in the mixture, causing it to become foamed. The surface active agent helps to control the size and size distribution of the bubbles so formed. As the curing progresses, the foamed mixture becomes increasing viscous until the mixture is fully expanded and cured, thereby forming the solid foamed phenolic material.
The phenol-aldehyde resin or resole used in the known foamable compositions may be prepared by reacting phenol, or a derivative of phenol such as cresol, with an aldehyde. Most typically, formaldehyde is used, but other aldehydes are used on occasion, such as acetaldehyde. This reaction is generally carried out in the presence of water and an alkaline catalyst such as sodium hydroxide or potassium hydroxide in an amount of between about 1 to 50%, and at a temperature within the range of between about 50.degree. C. to 125.degree. C., and for a time period of between about 1 to 24 hours. However different quantities of catalysts and different times for reaction may be used as well.
After the resin reaction product has reached the desired state of reaction, it is customary to remove a portion of the water formed by the reaction. This partial dehydration can be effected by application of a vacuum. It may also be desired, although not required, to reduce the alkalinity of the resin formed. This resin, also known as a resole, is then ready for incorporation in a foamable composition.
Typically, the acidic curing agent or catalyst added to the foamable composition to solidify the phenol-formadelhyde resin is an aqueous solution of a strong acid such as an aromatic sulfonic acid, hydrochloric acid, or a sulfuric acid.
A number of different foaming agents have been developed and used in the art. These include water, which vaporizes to steam during the exothermic curing reaction, and salts of carbonic acid, such as sodium hydrogen carbonate or potassium carbonate, which react with the acidic curing agent to liberate carbon dioxide gas. Organic compounds which produce gases when heated by the exothermic curing reaction, such as dinitrosopentamethylenetetramine, have also been used. Preferably, however, the foaming agent is a volatile organic compound, for example, a low-boiling hydrocarbon such as hexane, or a low-boiling haloalkane such as trichloromonofluoromethane, which compounds are vaporized by the heat given off during the exothermic curing reaction.
Tyically the surface active agent used to control the bubble size of the foam is a silicone glycol copolymer, such as a low molecular weight polydimethylsiloxane.
Filler or extender materials may also be added to the mixture to be foamed, which are generally relatively inexpensive materials used to extend the bulk or volume of the foamable composition, and which also may modify or impart certain desired characteristics to the foamed material produced. Typical examples of filler/extender materials include asbestors, clay, mica, glass fibers, talc and silica sand, or certain organic materials such as wood flour, cork and the like.
When compared to competing foams made from other thermoset resins, such as urethane foams or urea foams, phenolic foams have three principle characteristics which may be disadvantageous in certain applications. These are (1) phenolic foams have little or no self-skinning ability; (2) it is very difficult to consistently and repeatedly foam a phenol-formaldehyde resin to a predetermined foam density; and (3) phenolic foams have a relatively high proportion of open cells when compared to competing foam material.